1. Field of the Invention
This invention relates in general to new and useful improvements in a determination for the presence of water-soluble toxic reducible metals and metal salts in water and a method for removal therefrom and, more particularly, to a method for determining the presence of toxic metal salts and a system for the removal of such metal salts from drinking water which relies upon a reduction of the metal salt in a reduction/oxidation reaction to a different valence state and which would enable determination of the presence and allow for removal of the contaminant.
2. Brief Description of the Related Art
It is well established that water and, particularly, municipal drinking water, as well as water from aquifers and wells, presently contain trace amounts of highly toxic metals, including, for example, arsenic, mercury and chromium. Each of these metals are usually present in the form of various metal salts in different oxidized states and are known to be either carcinogens or otherwise known to present significant health risks.
Chromium is an example of a metal present as a metal salt contaminant found in many water sources. Two of the most common forms of chromium are trivalent chromium (Cr+3), and hexavalent chromium (Cr+6), although chromium can exist in other valence states, such as Cr+5, Cr+4 and Cr+2. It is also known that salts containing metals, such as Cr+3 are relatively harmless while salts of Cr+6 are highly toxic. Many of these metals and metal salts, such as Cr+6 and Cr+6 compounds are not normally present as natural constituents of environmental media, and their presence is almost always the result of human activity, including commercial and industrial processes, which generate Cr+6 and its salts and release them into the environment. As an example, cooling towers and ancillary equipment, catalysts used in the cracking and refining of petroleum products, tanning, textile dying, etc., are some of the commercial and industrial processes which give rise to these oxidized metals and their salts, such as those of Cr+6.
It is also known that many of these metals and their salts, such as Cr+6, which may be airborne, can induce lung cancer through inhalation. The presence of Cr+6 is not only recognized as a toxic substance, but its presence in drinking water is limited by current U.S. government standards with a maximum contaminant level of 0.1 milligrams per liter (100 parts per billion). In the State of California, USA, the standard for chromium in drinking water is 50 parts per billion. A present proposal suggests that even these standards are too lax and that the maximum allowable amount of total chromium present should be no more than 2.5 parts per billion. There have even been studies which suggest that the maximum amount of Cr+6 which should be allowable in drinking water should be no more than 0.2 parts per billion.
The seriousness of the health consequences of these oxidized metals and metal salts, when present in drinking water have been studied. It has been found that Cr+6, for example, can be distributed throughout the body and accumulates in the kidney, spleen and pancreas. Uptake of Cr+6 into the liver is 40 to 90 times that found in other organs. While Cr+3 does not readily enter the cells of these organs, Cr+6 does so. Within cells, Cr+6 is reduced stepwise to Cr+5, Cr+4, and Cr+3. During this process, aberrant forms of oxygen, including hydroxyl free radicals, OH, and the superoxide anion, O2xe2x88x92, are produced as potent toxins. These potent toxins can cause chemical changes in cellular DNA, i.e., mutations, leading to severe alterations in cell functions and carcinogenic effects. Other effects of these Cr+6-induced toxins result in a potent genotoxic agent. Still other effects of these Cr+6-induced toxins result in compromising the body""s immunoprotective systems and they can act as neurotoxins. In addition, they can cause developmental and reproductive damage, not to mention other adverse conditions and maladies caused by their presence.
Although it would be desirable to advise the public of the presence of metals and their metal salts in water, and particularly those which are toxic, no convenient test is currently available to detect their presence. Many of these oxidized metals are not visible at low concentrations, cannot be tested directly and easily and are otherwise sensibly indeterminable. Nevertheless, determination of the presence of metal toxic substances in water by the public in general would be desirable.
The prior art has usually involved the removal of Cr+6 and other toxic oxidizable metals by providing a reducing agent or reductant as a source of electrons. These agents reduce these metals to a lower valence state, one which is often non-toxic or less toxic. However, precipitation of Cr+6, for example, by control of pH alone is insufficient to remove chelated or complexed forms of Cr+6 or other heavy metals. There have been attempts to use controlled pH methods supplemented with flocculents or precipitants to allow for the removal of Cr+6 through flocculation, precipitation and settling out followed by filtration.
Reduction of Cr+6 to Cr+3 does eliminate the toxic hexavalent form of the metal by converting it to the essentially non-toxic form of Cr+3 by the reaction:                                           Cr            ⁢                          xe2x80x83                        ⁢            VI                    +                      3            ⁢                          xe2x80x83                        ⁢                          e              xe2x80x2                        ⁢            s            ⁢                          reduction                              ⇐                                                      xe2x80x83                                    oxidation                                                                    ⇒                  Cr          ⁢                      xe2x80x83                    ⁢          III                                    1        )            
Conversion of the Cr from a higher valence state to a lower valence state is accompanied by oxidation of the reducing agent or reductant from a corresponding lower valence state to a higher valence state. The combination of these two reactions constitutes a coupled redox reaction.
There have been several attempts and proposals for removal of some of these oxidized metal contaminants from water including, for example, U.S. Pat. No. 4,149,953 to Rojo, which relies upon an electrolytic cell to remove impurities. An anode of this cell containing aluminum particles and a cathode containing iron particles operates with the water serving as an electrolyte. The aluminum and iron particles which enter the water function as flocculents and adsorb impurities in the water. The flocculated materials are then separated from the water by conventional means. Application of this type of system to waste, process or drinking water for removal of Cr+6 or other oxidized metals is limited because it may bring down the chromium in the flocculate without necessarily reducing the metal. Moreover, this type of process would be difficult to implement, would not be efficient and would be costly to operate. Thus, the desirable conversion of Cr+6 to Cr+3 and subsequent removal of the latter as a precipitate, may not result.
U.S. Pat. No. 4,693,798 to Gale and O""Donnell discloses use of an electrolytic cell for generation of Fe+2 ions in an acidified medium. The Fe+2 interacts with Cr+6 ions reducing them to Cr+3 ions. However, a part of this stream is bled off into the contaminated stream containing Cr+6 ions. This must be followed by alkalinization of the treated stream of water to a pH in excess of 7.5 to allow for coprecipitation of Cr+3 and Fe+3 hydroxides.
Another system for waste water treatment is described in U.S. Pat. No. 4,923,599 to Bowers. In the system described in the Bowers patent, a controlled volume of waste water containing heavy metals and including, for example, Cr+6, is treated by optimizing and controlling pH to cause precipitation of some of the heavy metal contaminants. This is followed by filtration and monitoring of the samples"" turbidity to determine the amount of additional precipitating agent which may still be needed. Although a claimed advantage of this method is the reduction in the amount of resultant sludge, this system must be operated continuously so as to be most effective in achieving a predetermined set point of heavy metal concentration in solution. Moreover, it is not very effective and still leaves substantial amounts of trace heavy metals in the water.
U.S. Pat. No. 5,000,858 to Manning and Wells sets forth a method for removing hexavalent chromium from water which employs two or more reactors for treating waste water. Each reactor contains a flocculator and a clarifier for batch treatment in an acidic medium below a PH of 3. A reducing agent is added and the pH is thereafter increased to an alkaline pH where the water is then transferred to the second reactor having a flocculator. In this case, a flocculent is added and the solution is then transported to a clarifier where the heavy metal-containing solids are removed.
U.S. Pat. No. 5,000,859 to Suciu, et al discloses a process in which a sodium sulfide/ferrous sulfate treatment is used to remove hexavalent chromium, as well as other potentially toxic metals, from industrial waste waters. This patent discloses the use of sulfur dioxide, sodium sulfite, sodium bisulfite, sodium borobydride, and the use of ferrous ions as reducing agents. This method relies upon reduction of Cr+6 to Cr+3 in a pH range of about 7 to 9 by addition of ferrous ions from ferrous sulfate followed by sulfide or more ferrous ions to induce precipitation of the Cr+3 and of other reduced heavy metals. Polymers are added to aid flocculation of the reduced metals by formation of a precipitate or sludge and to clarify the waste stream.
U.S. Pat. No. 5,045,213 to Bowers also sets forth a waste water treatment for removal of metals by precipitation and filtration. The pH levels are optimized to precipitate the metals from the water samples and continuous sampling is used to determine presence of unprecipitated metals still remaining in solution. The metals in the water are treated with a Group II metal dithiocarbamate precipitating agent to precipitate chelated and/or complexed metal in the waste water stream unaffected by pH control. In addition, and in accordance with Bowers, it may be necessary to use a pre-treatment in order to reduce the amounts of the dithiocarbamate precipitants.
U.S. Pat. No. 5,370,827 to Grant, et al sets forth a method of solution decontamination in which heavy metal-containing water is treated with precipitants, such as sodium silicate and ammonium hydroxide. The pH of the contaminated solution is adjusted to about pH 5 to about pH 9.5 using hydrochloric acid. The resultant gels polymerize and/or precipitate the contaminant-containing silica matrix which forms a separable solid easily removed from the water by filtration.
U.S. Pat. No. 5,380,441 to Thornton describes a procedure for removal of chromium with mechanically agitated iron particles. The Cr+6 is converted to Cr+3 and precipitated with Fe+3 hydroxides. Solution pH is maintained in an acid range of 2 to 7 during the reduction of the chromium, and the pH thereafter is readjusted with base to facilitate the formation and precipitation of chromium hydroxide.
Although the removal of these toxic oxidized metals and metal salts is a problem which must be addressed by governmental agencies and industry, the consumer should at least be aware of the potential presence of these metals and metal salts in their drinking water. Thus, it would be desirable to provide a simple test to determine the presence of such highly toxic metals and metal salts. Even more so, it would be desirable to provide a process which is effective for the removal of Cr+6 and other oxidized metals from drinking water which can be performed at relatively low cost and high efficiency.
It is, therefore, one of the primary objects of the present invention to provide a visual determination which allows unskilled personnel to automatically and easily detect the presence of metal contaminants in water.
It is another object of the present invention to provide a method of allowing testing of drinking water for the presence of metals and metal salt contaminants visually by addition of a simple tablet thereto.
It is a further object of the present invention to provide a method for reduction of metals and metal salts to a reduced valence state allowing for precipitation of potentially toxic metals from a stream of water.
It is an additional object of the present invention to provide a method of providing a stream of drinking water through removal of Cr+6 and other oxidized metal contaminants by using a reduction reaction for reducing a metal contaminant in the presence of a reducing agent.
It is still another object of the present invention to provide a method of removal of oxidized metal constituents in water by the addition of a reducing agent in which a reduction/oxidation reaction takes place along with removal of the precipitated and reduced metal constituents, as well as co-reduction of the oxidized reducing agent.
It is still a further object of the present invention to provide both a process for testing and a process for removing metal contaminants from water in a relatively inexpensive but highly efficient manner.
With the above and other objects in view, my invention resides in the novel features of form, construction, arrangement and combination of parts and components presently described and pointed out in the claims.
1. Basic Principles of the Invention
The present invention first relates to a composition for the detection of oxidized metal contaminants in drinking water. In this respect, the term xe2x80x9coxidized metalxe2x80x9d will refer to metals and metal salts. In addition, the term xe2x80x9coxidizedxe2x80x9d will refer to those metals which are multi-valent and can exist in an oxidized state, that is have a higher valence state than other valence states for that metal. For example, Cr+6, the highest oxidized state of this metal, can exist in reduced valence states, such as Cr+5, Cr+4 or Cr+3.
The testing of water is preferably conducted with a three-component system, such that the three components may be added individually or together, in the form of a tablet, to a sample of water. In accordance with this aspect of the invention involving the detection of metals, such as hexavalent chromium, the first component is a reducing agent in the form of a water-soluble metal salt having the capability of existing in an oxidized metal state and in a reduced metal state. For example, iron in its ferrous form, Fe+2, provides electrons for the reduction of Cr+6 to Cr+3. Similarly, other metal constituents would be converted to their reduced metal salt forms. In these processes, ferrous iron, Fe+2, is oxidized and converted to ferric iron, Fe+3. Generally, for the conversion of Cr+6 to Cr+3, a water-soluble ferrous salt, like ferrous sulfate, may be used.
The second component in this three-component system is an acidifying substance, for example, citric acid or another tri-carboxylic acid. The acidifying component assures solubility of oxidized chromium salts present in drinking water and forms a stable complex with ferrous iron.
The third component is a mixing aid, such as sodium bicarbonate, which, in the presence of the acidifying component, facilitates the break-up of the tablet, and allows its components to rapidly diffuse throughout the sample of water being tested, thereby speeding up the redox reaction.
In accordance with this method, the oxidized metal, such as the Cr+6, for example, is reduced to Cr+3, while the Fe+2 in ferrous sulfate is converted to Fe+3. The products of these redox changes lead to co-precipitation of Cr+3 and Fe+3 in the form of insoluble mixed hydroxides. The change in valence states of the chromium and iron result in water turbidity and color changes which allow for the easy visual determination of the presence of the toxic metal constituent.
The present invention also provides a method of removal of heavy metal contaminants from water through essentially the same chemical process. In this case, the metal in the water is treated with a reducing agent involving minimal intervention. This is followed by removal of the precipitated metal when reduced and the re-reduction of the oxidized reducing agent.
More specifically, in connection with the method of treating water, a reducing agent is added in an acidified medium. Thereafter, and following a redox reaction involving reduction of the metal contaminant and coupled oxidation of the reducing agent, the pH would again be raised to an alkaline level through the addition of a base. Clarifying agents, such as flocculents and/or precipitants are then added to provide for the removal of the aggregated reduced and generally precipitated toxic metal constituent. In addition, electrons could be introduced through an electrical conductor into the treated water to reduce the oxidized reducing agent and to maintain the precipitated contaminant in a reduced valence state. This inflow of current (electrons) would be accompanied by an inflow of hydrogen ions (protons) along a separate conduction path to re-establish and maintain the pH in a desired acid range, for example, between 2.4 to 6.5.
In particular, the invention is primarily directed to, although by no means limited to, reduction of Cr+6 to Cr+3 in a stream of water by use of a reducing agent, such as Fe+2, e.g. ferrous sulfate. In the reduction reaction, the ferrous sulfate is converted to an oxidized form of iron by a transference of electrons, and the chromium is correspondingly reduced. The lower valence state of the metal contaminant obviously corresponds to the reduced state of the metal, and the higher valence state of the reducing agent is produced as a consequence of yielding electrons contributed by the metal atom of the reducing agent which then becomes oxidized.
Solubility of the metal contaminant while in its oxidized form, e.g. Cr+6, may be either higher or lower than the reduced form of that same metal salt. Obviously, if the solubility of Cr+6 is higher, the lower solubility of the reduced form of the salt (Cr3+) will facilitate co-precipitation with the oxidized metal constituent of the reduction agent. Cr+6 is known to have a higher solubility in aqueous media than Cr+3. The same holds true of the reducing agent, in that it may have lower solubility in its oxidized form. In the case of the reduced Cr+6, this co-precipitate may exist in the form of Cr3+ and Fe3+ mixed hydroxides at pH""s above 7.0. If the solubility of the reducing agent is higher, some of the oxidized form of the reducing agent salt will remain in solution where, after removal of the precipitated material, it can be easily re-reduced by electrons introduced into the reaction media.
The preferred reducing agents are generally selected salts of these metals and they must have the necessary solubility properties. The metal salt reducing agents must also involve metals capable of existing in two or more valence states, such as an oxidized metal atom state and a reduced metal atom state. The anions of the metal salt reducing agents may include acetates, chlorides and other halides, nitrates, sulfates, etc. Moreover, the hydrates of these salts are usually more soluble in water then their non-hydrated congeners.
2. Character and Advantages of the Invention
The invention can adopt the form of a cyclic process during which di-, tri-, and/or polyvalent metal salts acting as reducing agents convert toxic polyvalent metal contaminants, e.g. chromium (Cr VI), in water, including drinking water, to non-toxic lower valence state metal salts, e.g. chromium (Cr III). In this process, the metal salts of the reductants are oxidized while the chromium salts are reduced. The process is driven by a fuel cell that continuously regenerates these reducing agents, thereby making this a recyclable activity. This same process can be used to remove arsenic and mercury and their salts from various sources of water. Although the invention is operable with other metal components, it will be described in terms of conversion of Chromium VI to Chromium III.
Removal of the trivalent chromium (Cr III), and of the relatively small quantities of the oxidized reducing agents, as mixed insoluble hydroxides, necessitates the periodic replenishment of the small fraction of these reducing agents lost during this recycling process.
The advantages of this process derive from its simplicity, visual control, and recyclable nature. This insures that human intervention will be minimal. Personnel will not require extensive training to become effective operators of this system. Nor will their training require extended periods of time or expensive training materials to qualify and to be certified as operators. Replacement costs, including those of the metal salt reductants, and acids and bases used for automated and pre-programmed pH adjustments, as well as costs involved in operation and maintenance, will also be minimal.
The efficiency of the reducing process may be enhanced by mixtures of reductants, especially of salts of metals belonging to the same or closely related chemical groups in the Periodic Table.
The recyclable character of the overall process is dependent on three subordinate cycles: (1) the redox cycle of the metal salt reductants; (2) the automated and preprogrammed acidificationxe2x80x94alkalinization cycle to maintain selected pH ranges; and (3) operation of a fuel cell.
The preferred fuel for the fuel cell is hydrogen produced by the electrolysis of water. However, methane and/or other low molecular weight alkanes may be used to produce the hydrogen. The burning of these fuels in the fuel cell would produce a stream of protons for the control of pH. The accompanying but separate stream of electrons would facilitate the re-reduction of the oxidized reductants following conversion of Cr VI to Cr III, respectively, with minimal adverse environmental impacts. The streams of protons and electrons would be conducted directly from the fuel cell along two different conduction paths into the main chamber of the reactor.
This invention possesses many other advantages and has other purposes which may be made more clearly apparent from a consideration of the forms in which it may be embodied. These forms are shown in the FIGS. 1 and 2 forming a part of and accompanying the present specification. They will now be described in detail for purposes of illustrating the general principles of the invention. However, it is to be understood that the following detailed description is not to be taken in a limiting sense.